advance energy devices lion and supercap ch 2.pdf

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59

Advanced Energy Devices: Lithium IonBattery and High Energy Capacitor

M. K. Devaraju, M. Sathish, and I. Honma

Abstract

The development of modern technology toward energy production and storage

is essential to support human life with wide impact on the environment,

human health, and worlds economy. Through the development of the advanced

energy systems, human life can be ensure in a networked society even more

conveniently. In the electric and energy field, secondary batteries will play a

critical factor in reducing the environmental hazard and enable the effective

construction of the green energy society. At present, high power density and high

energy density are required as a power sources for the hybrid electric vehicle(HEV) and electric vehicle (EV). As we know, Li-ion battery has high energy

density but low power density. The energy density of Li-ion battery decreases

with the increase in rate capability, but electric double-layer capacitor has high

power density but low energy density. So, this chapter focuses on the advanced

energy devices such as lithium-ion battery and high energy capacitors beginning

with brief introduction.

The importance of the solution process mainly including the hydrothermal

and solvothermal method as sustainable chemistry toward the processing of

the positive electrode materials for lithium-ion batteries has been discussed.

The requirement and different techniques of the carbon coating using differentcarbon sources to improve the electrochemical property of the positive electrode

materials have been focused. The electrochemical property of the olivine-

structured cathode materials affected by different particles size and morphology

has been addressed. The concept of using graphene-based compounds for

the electric double-layer capacitor applications and electrochemical capacitor

M.K. Devaraju () M. Sathish I. Honma

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Aoba-ku,Sendai, Japan

e-mail:[email protected];[email protected];

[email protected];

J. Kauffman, K.-M. Lee (eds.), Handbook of Sustainable Engineering,

DOI 10.1007/978-1-4020-8939-8 105,

Springer Science+Business Media Dordrecht 2013

1149
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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1150 M.K. Devaraju et al.

based on pseudocapacitance has been discussed. The hybrid capacitors such

as metal oxide-doped graphene and PANI/graphene nanocomposites with their

electrochemical performances have also been discussed.

1 Introduction

The rapid growth of science and technology during the last several decades in the

world is keep changing the human life by contributing to their needs to enjoy

their living. Recently, the development of the advanced energy devices becomes

critical to global human development including the ecosystems, economic growth,

employment, and prosperity of the present and future generation. In addition, the

slow diminishing of available energy resources is an alarm to change the present

energy system to the sustainable and renewable one for a long-term energy supplyfor the mankind. Moreover, realization of the low carbon society based on

the advanced technologies for the sustainable development is one of the greatest

challenges at present.

The key technology for this challenge is to develop the next generation of clean

energy storage devices with high power density, high energy density, and high safety

for the hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and

pure electric vehicles (PEV) (Aric-o et al.2005). The practical realization of these

applications involves the development of the advanced energy functional materials

for the high energy storage and for a long-term performance. Lithium-ion batteryand capacitors are considered as the future advanced energy storage systems for

various next generation electronic and electrical applications.

Lithium-Ion Battery: Lithium-ion (Li-ion) batteries are comprised of cells that

employ lithium intercalation compounds as the positive and negative materials. As

a battery is cycled, lithium ions (LiC) exchange between the positive and negative

electrodes. They are also referred to as rockingchair batteries as the lithium ions

rock back and forth between the positive and negative electrodes as the cell is

charged and discharged. The positive electrode material is typically a metal oxide

with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material with

a tunneled structure, such as lithium manganese oxide (LiMn2O4), on a currentcollector of aluminum foil. The negative electrode material is typically a graphitic

carbon, also a layered material, on a copper current collector. In the charge/discharge

process, lithium ions are inserted or extracted from the interstitial space between

atomic layers within the active materials (Ehrlich2001).

The mechanism involved in Li-ion battery is shown in Fig. 59.1. Where the

lithium metal was substituted with other insertion compounds, such as graphite

or non-graphitic carbon, LiCoO2 was used as the cathode material. The entire

electrochemical process would involve the reversible transfer of lithium ions

between the two electrodes. During the charge process, lithium ion is de-intercalatedfrom the cathode layers, then transported and intercalated into the carbonaceous

anode. While the discharge process occurred, the lithium ions are deintercalated

from the carbonaceous anode and intercalated again to empty site between layers of

the cathode materials (Park2010).


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59 Advanced Energy Devices: Lithium Ion Battery and High Energy Capacitor 1151

e

e e

e

e

e

e

e

A

LixC6 +Li1xCO2

Li+

charge

dischargeLi

+

Fig. 59.1 Schematic representation of Li ion battery showing discharge intercalation mechanism

(Ehrlich2001)

Among the batteries, especially secondary batteries have been essential part of

the power source for the advanced energy devices. In the future, they will become

a key factor to pursue comfortable human life. The electric power source willbe produced by the environmentally friendly wind generation and solar cell, and

the produced electric power can charge the batteries which are not harmful and

friendly to the environment. In the electric and energy field, secondary batteries

will play a critical factor in reducing the environmental hazard and enable the

effective construction of the green energy society (Park2010). After three decades

of development in battery technology, the Li-ion battery technology has emerged

as one of the most popular battery technologies (Tarascon and Armand 2001).

They are widely used in various electronic devices because of their good cycle

life, high energy density, and high capacity over any other battery technologies.

The Li-ion battery technology that now dominates much of the portable batterybusiness has matured enough over the last 5 years to be considered for the short-

term implementation in the hybrid electric vehicles (HEV) and electric vehicles

(EV) applications (Tarascon and Armand2001).

High Energy Capacitors: The discovery of a so-called condenser, now referred

to as a capacitor that electric charges could be stored on the plate, was made in

the mid-eighteenth century during the period when the phenomena associated with

static electricity were being revealed. The embodiment known as a condenser is

attributed to Musschenbroek (Encyclopedia Britannica1926) in 1746 at Leyden in

the Netherlands, hence the name Leyden jar. The electrochemical capacitor wassupposed to boost the hybrid electric vehicle to provide the high or strong power

for acceleration and additionally allow the recovery of braking energy. The electric

double-layer capacitor consisting of a single cell with a high surface area electrode

material is loaded with electrolyte (Kotz and Carlen2000). The schematic diagram


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collectorpolarizing

electrodes

electrolyte

separator

collector

electric double layer

+

+ +

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

Fig. 59.2 Schematic representation showing basic structure of electric double layer (Kotz andCarlen2000)

of the double-layer electric capacitor is shown in Fig. 59.2. Electric double-layer

capacitors store the electric energy in an electrochemical double layer (Helmholtz

layer) formed at the solid/electrolyte interface. Positive and negative ionic charges

within the electrolyte accumulate at the surface of the solid electrode. The quantity

of ion removed from the electrolyte equals the charge developed on the electrodesurface. Therefore, the maximum energy stored in a capacitor is limited by the

capacitance of the capacitor, C, and the maximum operating voltage, V.

Recently, high-performance electrochemical energy storage systems are being

investigated as they are very important for the electric vehicles and hybrid electric

vehicles (Liu et al. 2010). Great efforts have been made to develop high-power

(10kWkg1) electrochemical capacitors (ECs) due to their faster charge and

discharge processes, in seconds, than those of batteries. In addition, electrochemical

capacitors have a longer cycle life as compared to batteries because no or negligibly

small chemical charge transfer reactions are involved.

When compared with those of other secondary batteries and Li-ion battery, it has

the following advantages: long cycle life, >100,000 cycles; some system up to 106;

good power density (under certain conditions, limited by IR or equivalent series

resistance (esr) complexity of equivalent circuit); simple principle and mode of

construction; cheap material (for aqueous embodiment); combines state-of-charge

indication, Q D CV; and can be combined for the rechargeable battery for the hybrid

application (electric vehicles). However, ECs suffer from a lower energy density

than batteries (Liu et al. 2010). The energy density can be improved by adopting

asymmetric (hybrid) systems; at present they have been extensively explored by

combining a battery like Faradic electrode (as energy source) and a capacitiveelectrode (as power source) to increase the operation voltage, which leads to a

notable improvement of the energy density of high-power ECs nearly to that of

batteries (Yoshino2004).


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In this chapter, solution-based synthesis and characterization of the olivine-

structured lithium metal phosphates (LiFePO4, LiMnPO4, and LiCoPO4) nanoma-

terials used as the positive electrode materials will be focused. The sustainable

chemistry process such as supercritical solvothermal method and solution processthat has been used for the materials synthesis is discussed. These methods can

be considered as environmentally friendly, rapid, and easy process for large-

scale synthesis or preparation of the advanced functional materials. High-energy

capacitors such as metal oxidedoped graphene, PANI/graphene nanocomposites,

and electrochemical capacitors based on pseudocapacitance have been discussed

along with their electrochemical property for the advanced energy systems.

2 Positive Electrode Materials for Li-Ion Battery

Among the battery components, the cathode materials are the one which are crucial

in determining the high power, safety, longer life, and cost of the battery that satisfies

the requirements of the larger battery system. These can be applicable to the electric

vehicles, power tools, energy storage equipment, and so on (Padhi et al. 1997).

There are various types of materials being used as the positive electrode materials

for the lithium-ion batteries as shown in Fig. 59.3 (Tarascon and Armand 2001).

The structural, chemical stability, availability of redox couples at a suitable energy,

specific capacity, operating voltage, and safety issues are the primary considerations,

and these properties are different among the positive electrode materials shownin Fig. 59.3. Lithium-based electrodes have four types of structure which have

lithium insertion voltage of above 3 V. They include layers of lithium metal oxides

such as LiCoO2, LiNiO2, LiCoNiO2, and LiMnNiO2; the zigzag layers structure

of LiMnO2; the three-dimensional spinel type, LiMn2O4 and Li1=2Mn3=2O4; and

the olivine structure of LiMPO4 (M=Fe, Mn, Co, and Ni). Recently, Li2MSiO4(M=Fe, Mn, and Co) based cathodes have been investigated, which are envisaged

as the potential cathode candidate for the high-power batteries (Dominko2010).

This is because of their overwhelming advantages such as high theoretical capacity

(>330mAhg1 which is possible while extracting more than one LiC ion per

formula unit), high thermal stability through strong SiO bonding, safety, cost-

effectiveness, eco-friendliness, and ease to synthesize. This chapter discusses the

olivine-structured LiMPO4cathode materials.

3 LiMPO4(M=Fe, Mn, Co, and Ni) as Positive Electrodes forLi-Ion Battery

In 1990s, LiCoO2 was commercialized by Sony. Since then, a series of excellent

candidates has appeared because of high cost and oxidative instability of LiCoO2foruse as the cathode material. In this regard, the layered rock salt systems, LixNiO2(0


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Li1xMn2yMyO4

Li1xCo1yMyO2

Li1xNi1yzCoyMzO4[M=Mg,AI,...]

LixMn1yMyO2[M=Cr, Co,...]

Vanadium oxides

[V2O5, LiV3O8]MnO2

Composite alloys[Sn(M)-based]

Carbons

Graphite

0 200 400 600

Capacity (A h kg1)

800 1000 3,800 4,000

[Sn(O)-based]

3d-Metal oxidea

Li-ionpotential

Li-metalpotential

Positivematerials

Negativ

ematerials

Positive material:

Negative material:

of Li ion

limited cycling)

Li metal

of Li metal

(

of Li ion

limited RT cycling

of Li metal

Nitrides LiMyN2

Polyanionic compound [Li1xVOPO4,LixFePO4]

4

3

2

PotentialversusLi/Li+(

v)

1

0

Fig. 59.3 Voltage vs. capacity of some cathode materials (Tarascon and Armand2001)

to some degree. However, they are still problematic due to their compositions.

To overcome these disadvantages and problems, the olivine-structured lithium metal

phosphate cathode materials are considered as the attractive cathode materials

because they are in low cost, abundant in nature, exhibit high theoretical capacity

(170mAhg1) (Padhi et al.1997), high thermal stability owing to the presence of

a strong PO covalent bond, and easy to synthesize.

LiMPO4 (M=Fe, Mn, Co, and Ni) belongs to the orthorhombic structure, which

consists of the hexagonal closed packing (HCP) of the oxygen atoms with LiC and

M2C cations located in half of the octahedral sites and P5C cations in one eighth of

the tetrahedral sites(Fig.59.4). This structure may be described as chains (along the

c direction) of edge sharing MO6octahedra that are cross-linked by the PO4groupsforming a three-dimensional network. Tunnels perpendicular to the [010] and [001]

directions contain octahedrally coordinated LiC cations (along the b-axis), which

are mobile in these cavities (Jin and Jiang2009). Among the phosphates, LiFePO4is considered most stable, low cost, and high compatibility with the environments.

3.1 Synthesis of LiMPO4(M=Fe, Mn and, Co) Cathodes by theSolution Process

Since the demand for the cathode materials for lithium-ion battery is increasing

continuously, various methods have been developed to prepare lithium metal

phosphate nanoparticles, such as the solgel method (Choi and Kumta 2007),


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Fig. 59.4 The schematic representation of the crystal structure of LiMPO4 (M=Fe, Mn, Co, and

Ni) compounds showing the HCP oxygen array with MO6 and PO4 groups (Jin and Jiang2009)

coprecipitation (Arnold et al.2003), mechanochemical activation (Kim et al.2007),

spray technology (Konarova and Taniguchi 2009), and solid-state reaction (Padhi

et al.1997). However, all these methods have limitations in practice. Commercial

success of new cathode materials is mainly dependent on the preparation method,

which controls morphology, particle size, and cation order among other critical

experimental parameters. Although traditionally high-temperature methods have

been used, they are both energy intensive and cannot readily produce manypotentially metastable structures that might result in high lithium-ion diffusivity.

However, they do have the advantage of being hydroxyl/water-free. There are

many possible approaches for the synthesis of active materials, but in the end, a

commercially viable approach must be used (Whittingham et al. 2004). Therefore,

simple solution process could be required to overcome practical problems.

Recently, soft chemical approaches such as hydrothermal and solvothermal or

ion exchange offer several advantages and are being used on the tonnage quantities,

as such, the chemical industry considers them viable. Hydrothermal synthesis has

been extensively studied for the synthesis of simple oxides such as those of tungsten,molybdenum, and vanadium, and today many of the key parameters are understood

(Chirayil et al. 1997). Different kinds of phosphates have also been successfully

prepared by the hydrothermal method (Byrappa et al. 2008). The advantages of


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Precursors Spheres Plates Cubes Rods

Hydrothermal and solvothermal process

Hydrothermal and solvothermal products

PrecursorsTeflonliner

AutoclaveFURNACE/OVEN

Fig. 59.5 Showing experimental scheme of hydrothermal and solvothermal process for the design

of nanoparticles with various shapes (Author unpublished work)

using the hydrothermal and solvothermal methods are shown in Fig. 59.5, where

morphology, size, and other experimental parameters can be easily controlled.

Moreover, low-temperature solution process is environmentally friendly, low energy

consuming, and easy to perform for the large-scale production.

The concept of the hydrothermal process for the preparation of LiFePO4 was

first realized by Yang et al. in (2001). However, preparation of the nanocrystalline

particles ranging from 500 to 1,500 nm was reported by Tajimi et al. (2004) under

the hydrothermal reaction condition at the reaction temperature of 150220C for

several hours using various amounts of polyethylene glycol (PEG).

Recently, monodispersed particles of LiFePO4 were prepared using a mixture ofisopropanol and aqueous solution by controlling the RH factor (Zhang et al.2009).

The monodispersed LiFePO4 particles were controlled from 1 to 4m in length

and 12m in diameter. Small particles of LiFePO4 measuring 4001m with

the shape of discrete short rods were obtained at the RH factor of 1 ( Fig. 59.6a).

However, several attempts have been made to obtain cathode materials with different

size and morphology. In this regard, the rodlike LiFePO4 particles with 100 nm

of uniform diameter and 510 nm of the aspect ratio were synthesized by the

hydrothermal reaction at 220C (Huang et al. 2010), and the rodlike morphology

can be seen inFig. 59.6b.In order to reduce the reaction time and to obtain homogeneous monodispersed

particles, a few attempts have been made to synthesize the cathode materials at a

shorter reaction period. Hence, a rapid production of the cathode materials by the


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Fig. 59.6 LiFePO4 cathode materials with different morphology synthesized by hydrothermalmethod (Reproduced with permission from Zhang et al. (2009), Huang et al. (2010) and Xu et al.

(2008))

continuous hydrothermal method has been adopted to synthesize LiFePO4nanopar-

ticles (Xu et al.2008) ranging from 20 to 40 nm in diameter as shown inFig. 59.6c.

This process is promising for the large-scale synthesis of high purity monodispersed

nanocrystalline electrode materials for the application in the Li-ion battery.

Recently, surface modified LiFePO4/C nanocrystals were synthesized from the

supercritical batch reactors at 400C for 30 min using inexpensive environmentally

friendly solvent such as ethanol and water (Rangappa et al. 2009). The as-

synthesized particles exhibited less than 15 nm in diameter. Morphology can also beeasily controlled by this process such as the nanoarchitectured structure of LiFePO4as shown inFig. 59.7a (Rangappa et al. 2010), and nanoplate/nanorods of LiMnPO4(Fig. 59.7b, c) were synthesized in the presence of surfactants such as oleic acid and

oleylamine at 400C for 410 min (Rangappa et al. 2010a). In addition, LiCoPO4nanoparticles were also synthesized by this method. The as-synthesized LiCoPO4particles showed 50100 nm in diameter (Fig. 59.7d). The experimental results

confirmed that using selective surfactants and solvents, morphology and size of

the cathode materials could be easily modified. Furthermore, the reaction time and

temperature can be shortened in the light of the energy savings.

The microwave-hydrothermal and microwave solvothermal methods have beenused to synthesize the positive electrode materials within a short period of reaction

time as these methods have advantage of changing the reaction kinetics while

irradiating microwave. The platelets like LiMnPO4 nanoparticles (Ji et al. 2011)

were synthesized using the microwave hydrothermal method at 120180C for

a short period of reaction time, and the as-synthesized particles are shown in

Fig. 59.8a.The size of the platelets was controlled to 150 nm thickness and 5 m

basal dimensions. The nanorods of LiFePO4 were prepared by the rapid microwave

solvothermal method (Murugan et al.2008) at 300C for 5 min. The large nanorods

of LiFePO4with a width of 40 6 nm and a length of up to 1m were successfullysynthesized in presence of tetraethylene glycol (TEG) as shown in Fig. 59.8b.

This result showed the effect of selective solvents on the morphology of the final

products.


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Fig. 59.7 (a) Nanoarchitectured of LiFePO4 (b) and (c) nanoplate and nanorods of LiMnPO4and (d) sphere-like particles of LiCoPO4 synthesized by supercritical method (Reproduced with

permission from Rangappa et al. (2010a) and Rangappa et al. (2010b) and LiCoPO4 from authors

unpublished work)

Polyol-mediated solvothermal synthesis (Lim 2010) of LiMPO4 (M=Fe and

Mn) particles with dimensions of length and width in the range of 200350 and

200400 nm, respectively, are shown in Fig. 59.8c.The use of the polyol solvents

acts not only as a solvent but also as a reducing environmental agent and stabilizer,

thereby limiting particle growth and preventing agglomeration. Thickness of the

plate-like LiFePO4 down to 30 nm, width of 100 nm, and length of 200 nm were

synthesized by the solvothermal method at 180C for 18h (Nan et al. 2011).

Morphology and well-resolved lattice fringes of LiFePO4 can be seen inFig. 59.8d.Morphology and size have their own effects on the property of the cathode materials.

Therefore, the solution-based green synthesis methods are much beneficial in

designing the nanocrystalline electrode materials since the nanocrystalline materials


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Fig. 59.8 (a) Platelet like of LiMnPO4 nanoparticles synthesized by microwave hydrothermalmethod (b) large nanorods of LiFePO4 synthesized by microwave solvothermal method (c)

platelike LiFePO4 synthesized by polyol method (d) thin plate like LiFePO4 synthesized by

solvothermal method (Reproduced with permission from Ji et al. (2011), Murugan et al. (2008),

Lim et al.(2011) and Nan et al. (2011))

possessing different shape are promising to behave much differently than the

bulk materials. Hence, from the nanocrystalline materials, improved and surprising

physicochemical reaction can be expected, which could play a vital role in various

technological and chemical applications.

3.2 Conductive Coating of Cathode Materials

The practical capacity and high rate performance of the olivine-structured lithium

metal phosphates are not impressive due to its low electronic conductivity


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(1 109 scm1 at room temperature). To improve the electronic conductivity

of these materials, the following approaches have been considered:

(a) Decreasing the particle size for shortening the ionic and electronic transport

(b) Surface modification by the conductive carbon coating(c) Doping the supervalent cations to enhance the intrinsic conductivity

(d) Designing different morphology and changing the texture and pore size

However, the expected electronic conductivity cannot be reached simply by

producing the cathode materials with different morphology, size, or doping su-

pervalent cations. Therefore, surface modification of these materials is necessary

by conductive carbon coating because carbon coating offers better electronic

conductivity. The conductive carbon coating can be done either by the in situ or

ex situcoating technique. The in situ coating involves mixing of the carbon source

during the synthesis of the cathode materials at various reaction temperatures. This

method is useful to achieve homogeneous coating around the individual particles.

However, sometimes coating may not be uniform due to inhomogeneous mixing or

improper design of the experimental conditions. Theex situcoating involves mixing

of the carbon source either by the physical mixing or by high-energy ball milling

method.

Recently, various conductive carbon sources were successfully coated on to the

cathode materials, and this chapter discusses several carbon coating techniques.

Sugar was used as the carbon source by Jeon et al. (2007), and the in situ mixing

was carried out during the synthesis of the LiFePO4 particles, and carbon was

successfully mixed with the cathode materials as shown in Fig. 59.9a. Under thesupercritical water conditions, ascorbic acid was used as the carbon source, and the

in situ coating was carried out to synthesize the LiFePO4/C particles (Rangappa

et al. 2009). Formation of the carbon layer can be seen in Fig. 59.9bfor the heat

treated LiFePO4 particles at 500C. This result shows that ascorbic acid can act as

the reducing agent to prevent the oxidation of metal ion from divalent to trivalent

and also as effective carbon source for the conductive coating.

Saravanan et al. (2009) have used D-gluconic acid lactone (C6H10O6) as the

carbon source, and the in situ coating was carried out using the solvothermal

method. Figure 59.9c shows 5-nm carbon layer formation around the particles.

This result shows that at low temperature (250C), the carbon coating can be

achieved using the selective carbon sources. The conductive polymer such as

PEDOT was used to coat LiFePO4by Murugan et al. (2008) via the rapid microwave

solvothermal method. The homogeneous conductive polymer coating can be seen

around the LiFePO4 nanorod as shown inFig. 59.9d.

Hence, better carbon coating can be made through the solution process as all of

the solution routes start from a precursor in a liquid solution which provides intimate

mixing of the carbon source with the ingredients on the atomic level, leading to

the rapid homogeneous nucleation and uniform particle formation. Moreover, when

compared with the other methods such as the carbonothermal and high-temperatureheating process used for the carbon coating and materials preparation, the solution-

based synthesis methods have advantages on the materials preparation and the

carbon coating at the low temperature with the selection of suitable organic solvents


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Fig. 59.9 Carbon-coated LiFePO4materials using (a) sugar, (b) ascorbic acid, (c) D-gluconic acidlactone, and (d) PEDOT polymer (Reproduced with permission from Jeon et al. (2007), Rangappa

et al.(2009), Saravanan et al. (2009) and Murugan et al.(2008))

which can contribute as carbon source during the synthesis of the cathode materials.

High-boiling point solvents like glycols can be beneficial as the carbon source and

to achieve better in situ coating at the time of synthesis.

3.3 Charge/Discharge Process ofCathodeMaterials

Specific capacitance of the potential Lithium-ion battery cathode material is usu-

ally determined by the electrochemical galvanostatic charge/discharge technique.

Specific amount of the cathode materials was mixed with the carbon additives


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1.2

2.0

2.5

3.0

3.5

1.3 1.4

VoltageversusLi/Li+(

V)

1.5 1.6

Charge

Voltage gap resulting from hysteresis

Discharge

xin LixFeSiO4

1.7 1.8 1.9 2.0

Fig. 59.10 Typical galvanostatic chargedischarge curve of LiFeSiO4at a C/20 rate (Reproducedwith permission from Dreyer et al. (2010))

and then with the binder to make electrode paste. The electrode paste will be

pressed on Ni mesh and then cell assembled in the argon glove box. For the

charge/discharge measurement, the current rates of the cathode materials will be

calculated based on its theoretically calculated specific capacity in various timespans and considering the amount of the cathode material present in the electrode

paste. The cut-off voltages used for the measurements are adapted according to the

type of the measured cathode material. It is important to note that materials that can

be cycled at high C-rates and operating in the voltage window of commercially used

electrolytes are desirable. During the galvanostatic cycling, structural change occurs

inside the electrode material as a result of the LiC insertion/extraction (lithium metal

is used as a counter electrode anode). The results from the measurements can be

plotted in the galvanostatic charge/discharge curves (voltage vs. capacity, voltage

vs. composition as shown inFig. 59.10(Dreyer et al.2010), and capacity vs. cycle

number). Important parameters that can be determined from these curves are specificcapacity, voltage, and reversibility (e.g., polarization, voltage gap between charge

and discharge).

The LiMPO4 (M=Fe and Mn) cathode materials with similar crystal structure

synthesized by various solution routes show different electrochemical property.

The discharge capacity is not only dependent on the structure of the cathode

materials, but other facts also influence the electrochemical property of the cathode

materials, for example, crystal size, morphology, and method of carbon coating. For

example, the charge/discharge profile of the LiFePO4nanorods shown inFig.59.10a

shows the discharge capacity of 140 mAhg1

(Fig. 59.11a) for the first cycle anddecreases to 130 mAhg1 after 20 cycles (Huang et al. 2010). The potential voltage

gap between the charge and discharge profile is wide. This might be due to low

conductivity, and it can be overcome by proper conductive carbon coating to


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20 0

0 60 120

1

1

50

50

25

25

Capacity (mAhg1

)

a

c

b

d

CellVoltage(V

vs.

Li/Li+)

180

1.0

4.5

4.0

3.5

2.5

3.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1.5

2.0

2.5

2.5

3.0

3.0

3.5

3.5

4.0

4.0

4.5

4.5

5.0

20 40

Voltage(V)

Voltage(V)

60 80 100 120 140

0.5C

First cycle

160 0

0

20

20 30

Charge-discharge at 0.1C

50 7010

40

40

60

60

80

Capacity (mAh/g)

Capacity (mAh/g)

Voltage(V

vs.

Li/Li+)

100 120 140 160180

Fig. 59.11 Chargedischarge curves of (a) LiFePO4 nanorods, (b) sphere-like LiFePO4/C,

(c) thin plate like LiFePO4/C, and (d) colloidal nanocrystal of LiMnPO4 (Reproduced with

permission from Huang et al. (2012), Rangappa et al.(2009), Saravanan et al.(2009)and Rangappa

et al.(2010a))

improve the electrochemical property. Discharge capacity of about 165 mAhg1

(Fig. 59.11b) for a sphere-like LiFePO4/C particle is synthesized by supercritical

water in the presence of ascorbic acid as the carbon source and reducing agent(Rangappa et al. 2009). The discharge capacity of these materials is close to the

theoretical capacity of LiFePO4 (170 mAhg1). The cyclic performance of these

material was quiet satisfactory. However, further study is necessary to reduce the

wide potential observed in the chargedischarge profile and to increase the high rate

performance.

The thin platelike LiFePO4/C (Saravanan et al. 2009) showed the discharge

capacity of around 150160mAhg1 up to 50th cycle as shown inFig. 59.11c.The

chargedischarge profile of this material shows relatively low potential gap between

the charge and discharge profile. This is due to the platelike morphology, whichcan provide short distance for the Li-ion insertion and exertion and also due to the

homogeneous coating of carbon around the platelike LiFePO4/C particles. The col-

loidal nanocrystal of LiMnPO4with rodlike morphology (Rangappa et al. (2010a))


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shows the discharge capacity of around 70 mAhg1 (Fig. 59.11d). The development

of new strategy of conductive carbon coating can enhance the discharge capacity

of LiMnPO4 materials. The intrinsic electronic conductivity of LiMnPO4 is very

low due to the JahnTeller distortion, and it is not possible to achieve fulltheoretical capacity of LiMnPO4. However, further study is necessary to improve

the electrochemical property of the LiMnPO4cathode materials.

4 Electrochemical Capacitors

Electrochemical capacitors, also called supercapacitors, store energy using either

ion adsorption (electrochemical double-layer capacitors) or fast surface redox

reactions (pseudocapacitance). It is well known that the electrochemical double-layer capacitors (EDLC) store the opposite charge electrostatically using reversible

adsorption of ions of the electrolyte on active materials surface that is elec-

trochemically stable and has high accessible surface area (Conway 1999). As

there is no chemical reaction involved in the storage mechanism, the process is

highly reversible for millions of cycles and results long life for the capacitors.

And the specific capacitance depends on the available active specific surface

area of the active materials. Whereas, in the pseudocapacitance, surface or near

surface redox reactions occur during the charge storage mechanism, resulting high

specific capacitance with relatively short cycle life compared to EDLCs. Thus, the

development of high capacitive energy storage systems with optimum cycle life, low

cost, and environmentally friendly materials is essential to meet the energy demands

of modern society and emerging environmental concerns.

4.1 Graphene-Based Electrochemical Double-Layer Capacitors

Carbons and carbon-based composites materials are the most widely used owing

to their high surface area, moderate cost, and ecofriendly nature. A variety of

carbon morphologies with different surface area and chemical nature such as carbonnanotubes, carbon nanofibers, carbon fibres, onions, and nanohorns have been

investigated (Simon and Gogotsi 2008; Zhang et al. 2009). Similarly, activated

carbons, mesoporous carbon, template carbon, and chemically derived carbon have

been examined. The carbons used in EDLC are generally pretreated to remove

moisture, and most of the surface functional groups are present on the carbon surface

to improve stability during cycling.

The presence of functional groups will result in increased serious resistance and

capacitance fading during aging. The double-layer capacitance of activated carbon

reaches 100120 F/g in organic electrolytes; this value can exceed 150300 F/g inaqueous electrolytes, but at a lower cell voltage because the electrolyte voltage

window is limited by the water decomposition. The research on carbon materials

was directed toward increasing the pore volume by developing high surface area


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carbon and refining the activation process. However, the capacitance increase was

limited even for the most porous samples (Simon and Gogotsi2008). From a series

of activated carbons with different pore sizes in various electrolytes, it was shown

that there was no linear relationship between the surface area and the capacitanceowing to the inability of electrolyte access to the entire surface through smaller

pores. Also, the poor conductivity of the porous carbon materials limits the high

capacitance. Thus, high surface area carbon material with optimum pore size and

conductivity is essential to realize the high capacitance.

The key to reaching high capacitance by charging the double layer is using high

surface area carbon materials with electronically conducting electrodes. Graphitic

carbon satisfies all the requirements for this application, including high conductivity,

electrochemical stability, and open porosity that offer huge surface area (Simon

and Gogotsi 2008). Thus, graphene-based supercapacitors are under intensive

investigation as potential alternative to the activated carbon that is used in thecurrent supercapacitor electrodes. The effective surface area of graphene materials

should depend highly on the number of layers, that is, single- or few-layered

graphene sheets with less agglomeration might be expected to exhibit higher

effective surface area. The recent research reports on clean graphene materials

with specific capacitance ranging from 120 to 250 F/g. The chemical nature of the

graphene nanosheets and its purity greatly depend on the method of preparation and

subsequent processing of the resulting graphene sheets.

As mentioned earlier, the presence of functional groups on the graphene surface

greatly influences on its capacitance. In general, the presence of oxygen containingfunctional groups result diminishes the capacitance and cycle life (Pandolfo and

Hollenkamp2006). Wang et al. (2009) reported a maximum specific capacitance

of 205 F/g with an excellent long cycle life along with 90% specific capacitance

retained after 1,200 cycle tests. Also, it is confirmed that the interfacial capacitance

of the multilayer graphene sheets is found to depend on the number of layers. In

addition to the graphene quality, the fabrication of electrodes and its structure also

influences the performance of the resulting supercapacitors. Recently, P.M. Ajyan

group reported an in-plane fabrication approach for ultrathin supercapacitors

based on electrodes comprised of pristine graphene and multilayer reduced graphene

oxide (Yoo et al.2011). And this approach allows for the formation of an efficientelectrical double layer by utilization of the maximum electrochemical surface area

and results a maximum specificapacitance of 247 F/g. The research on supercapac-

itors using graphene nanosheets is under tremendous progress. Combining high-

quality graphene sheets with suitable electrode fabrication technique will lead to the

commercial production of EDLC capacitors with high capacitance in the near future.

In the row of various chemical modification processes, doping of hetero-atoms,

such as nitrogen, sulfur, and boron, into the graphene backbone is another possible

route. Also, the heteroatom doping in the carbon materials (CNTs, graphene)

and metal oxides has always created excitement in the material chemistry as thematerials property can be tuned significantly. Recently, studies have focused on

direction, and several possible routes have been identified for the effective N- or

B-doping in the graphene sheets (Hulicova et al. 2006; Kwon et al.2009).


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Significant enhancement in capacitance has been reported for the heteroatom-

doped graphene sheets, and the following possible mechanisms have been proposed

for the enhancement: (1) the improvement of the electrode wet-ability, due to the

increase in the number of hydrophilic polar sites; (2) the decrease of equivalentseries resistance (ESR) of a capacitor cell by the increase of the carbon electric

conductivity; (3) the occurrence of space-charge-layer capacitance in carbon by the

increase of its electron density; and (4) the occurrence of that pseudocapacitance

through the Faradic charge transfer, because the nature of carbon becomes electron

donor. Though, it is difficult to point out one particular effect for the capacitance

enhancement, it is believed that the pseudocapacitance through Faradic charge

transfer is the most important factor in enhancing the capacitance of the N- or

B-doped graphene sheets. Since the research on this direction is still in infancy stage,

additional experiments and theoretical understating are necessary. Understanding of

the mechanism and further development in the preparation method is expected toplay an important role in the improvement of the supercapacitor performance in the

coming years.

4.2 Electrochemical Capacitors Based on Pseudocapacitance

The large specific pseudocapacitance of Faradaic electrodes (typically 3001,000

F/g) exceeds that of the carbon-based materials using double-layer charge storage,

resulting in great interest in these systems. Specific capacitance of more than 600 F/ghas been reported for the RuO2-based system owing to its good conductivity,

fast and reversible electron transfer together with the electroadsorption of protons

on the surface. However, the Ru-based aqueous electrochemical capacitors are

expensive, and the 1-V voltage window limits their applications to small electronic

devices (Simon and Gogotsi2008). Thus, pseudocapacitive transition-metal oxides

such as MnO2, NiO, and redox polymers such as polyanilines, polypyrroles, and

polythiophenes could be used to make electrodes, because they are predicted to

have a high capacitance for storing electrical charge, inexpensive, and not harmful

to the environment. Poor conductivity and lack of stability during cycling are

major drawbacks associated with these materials for their usage in supercapacitors.Thus, numerous efforts have been made to use these metal oxides successfully

in supercapacitors by making composite with conductive support such as carbon

and gold.

Recently, Lang et al. (2011) developed a nanoporous gold/MnO2 electrode by

combining chemical de-lloying Ag65Au35 (at %) with the electroless plating of

MnO2 (Fig. 59.12), in which nanoporous gold acts as a double-layer capacitor and

also provides good electronic/ionic conductivity to enhance the pseudocapacitive

behavior of the nanocrystalline MnO2. And the MnO2loading can be controlled by

adjusting the platting time. The gold/MnO2 hybrid material has very high specificcapacitance of 1,145 F/g at a scan rate of 50 mV/s. The obtained high specific

capacitance at a scan rate of 50 mV/s is higher (one order of magnitude) than

the reported MnO2 film electrodes at 5 mV/s (Toupin et al. 2004). This could be


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MnO2

nanocrystals

Growth

N2H4

N2H4Mn7+

NPG

Fig. 59.12 Schematic showing the fabrication process for nanoporous gold/MnO2 hybrid mate-rials by directly growing MnO2 onto nanoporous gold (Reproduced with permission from Lang

(2011))

attributed to the porous metal/oxide structure, in which the nanocrystalline MnO2grows epitaxially into the internal surface of the highly conductive nanoporous gold,

allowing easy and efficient access of both electrons and ions so as to afford a fast

redox reaction at high scan rates as well as good cyclic stability. The power and

energy densities of the hybrid structure increase with the loading rate of MnO2and

reach maximum of 57 Wh/kg and 16 kW/kg, respectively, with the MnO2 plating

time of 20 min. The high specific capacitances, charge/discharge rates, and good

cyclic stability offered by this hybrid structure make them promising electrodes

materials in supercapacitors.

4.2.1 GrapheneMetal Oxide NanocompositesAs the graphene nanosheets have vast surface area with excellent conductivity, it

will be an appropriate candidate to accommodate a large amount of metal oxides.

In addition, the EDLC behavior of the graphene nanosheets can also be enhanced,

which contributes to the total capacitance of the resulting composite. Recently,

graphenemetal oxide nanocomposite systems have been developed by various re-

searchers, and high specific capacitances with good cycling performance have been

reported (Zhang et al. 2009,2010; Huang et al. 2012;Simon and Gogotsi2008).

Also, layered double hydroxides (LDH) materials containing transition metals have

been reported to be promising electrode materials for supercapacitors because of

their relatively low cost, high redox activity, and environmentally friendly nature.

Gao et al. reported the preparation of graphene Ni/Al layered double hydroxide

(LDH) nanocomposite and a maximum specific capacitance of 781.5 F/g with an

excellent cycle life (Gao et al. 2011). The observed capacitance is almost 1.5 times

higher than that of the pure LDH electrodes. The larger capacitance for GNS/LDH

may be caused by the combination of electric double-layer capacitance and Faradicpseudocapacitance. At the same time, the open structure system of GNS/LDH

improves the contact between the electrode materials and the electrolyte and thus

makes full use of the electrochemical active material contribution to the overall


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capacitance. Development of suitable GNSmetal oxide or LDH nanocomposites

with appropriate ratio will lead to high capacitance owing to the combination of

EDLC and pseudocapacitance from GNS and metal oxide or LDH, respectively.

4.2.2 GraphenePolymer NanocompositesTo exploit the potential of the graphene-based materials for the supercapacitor

applications, graphene-conducting polymer nanocomposite was prepared by several

preparation route, and electrochemical capacitance was reported in the range of

233 500 F/g (Zhang et al.2009). However, the capacitance was mainly dominated

by the pseudocapacitance from the polymer films coated on the graphene paper

surface, and the EDLC from the graphene sheet was less utilized due to the agglom-

erated layer-like structure in the graphene paper. Among the conductive polymers,

carbon (or) graphenePANI composites have been extensively studied and well

documented in the literature for supercapacitor applications (Zhang et al. 2010;Huang et al. 2012). And flexible nanoelectrodes also have been developed using

carbon nanotube/PANI or graphene/PANI nanocomposites for the supercapacitors

applications (Meng et al.2010).

Typically, in all these studies, the aniline polymerization on the graphene surface

was carried out using oxidants such as ammonium persulfate ((NH4/2S2O8) or

ferric chloride (FeCl3), and the experimental strategy plays a vital role on the

morphology of the graphene/PANI composite and their electrochemical response.

Recently, preparation of graphenepolyaniline nanocomposite electrodes via oxida-

tive polymerization of aniline by MnO2was shown. And a superior supercapacitiveperformance (641 F/g, 1540% enhancement than the reported capacitance for

graphenepolyaniline) has been observed (Sathish et al. 2011). As mentioned

earlier, the method of polymerization plays a vital role on the materials property.

GO/MnO2 composite was prepared (Sathish et al.) by mixing appropriate amount

of MnO2 nanosheets and GO nanosheets. Then, appropriate amount of aniline

was added to the above composite, and the chemical oxidation polymerization of

aniline was initiated by the reduction of Mn4C ion, and the resulting Mn2C ions

will go to the solution. This process enables the formation of slow and uniform

polyaniline nanofibers on the graphene surface with significant porosity, which

enables the impulsive peculation of electrolyte to access large surface area (authorsunpublished work). Thus, the graphene surface also has been used for EDLC

in addition to the pseudocapacitance from polyaniline. Figure 59.13 shows the

schematic representation of the polyaniline formation on the graphene sheets.

Similarly, Xu et al. (2010) introduced a facile method to construct the hierarchical

nanocomposites by combining the one-dimensional (1D) conducting polyaniline

(PANI) nanowires with the 2D graphene oxide (GO) nanosheets. It is shown

that the aniline concentration plays a key role on the PANI morphology, at

lower concentration (0.05 M); vertically aligned PANI nanowire arrays on GO

surface are observed owing to the heterogeneous nucleation on the GO nanosheets(Fig. 59.14a). When aniline concentration was increased to 0.06M, homoge-

neous nucleation will take place after the initial nucleation on the solid surface.


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Mn2+Graphene oxide

GO-PANI

GO-MnO2

(i) Aniline

Mn4+

MnO2nanosheets

Fig. 59.13 Schematic representation of polyaniline formation on graphene surface via oxidative

polymerization of aniline by MnO2 (Reproduced with permission from (From authors work))

Consequently, random connected PANI nanowires were produced (Fig. 59.14b).

The hierarchical nanocomposite structures of PANI/GO were further proved by the

UV-vis, FTIR, and XRD measurements. The hierarchical nanocomposite possessed

higher electrochemical capacitance of 555 F/g at a discharge current density of

0.2 A/g and better stability than each individual component as the supercapacitor

electrode materials, showing a synergistic effect of PANI and GO. Also, the

observed specific capacitance of the nanocomposite is much higher than that of the

random connected PANI nanowires (298 F/g) obtained under the same condition.

This study will further guide the preparation of functional nanocomposites by

combining different dimensional nanomaterials.

4.3 Asymmetric (or) Hybrid Capacitors

Asymmetric or hybrid systems offer an attractive alternative to the

conventional pseudocapacitance or EDLCs by combining a battery-like electrode

(energy source) with a capacitor-like electrode (power source) in the same cell

(Simon and Gogotsi 2008). The high specific capacitances, cell voltage, and

charge/discharge rates offered by such hybrid structures make them promising

candidates as the electrodes in supercapacitors. MnO2graphene composite elec-

trodes have been developed for the high-voltage hybrid electrochemical capacitorbased on graphene as the negative electrode and MnO2graphene composite

as the positive electrode in the neutral aqueous Na2SO4 solution as electrolyte


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Nucleation

Growth

a b

Growth further

Aniline

Aniline anion

GO sheet

Fig. 59.14 Schematic illustration of nucleation and growth mechanism of PANI nanowires:(a) heterogeneous nucleation on GO nanosheets; (b) homogeneous nucleation in bulk solution

(Reproduced with permission from Xu (2010))

0.0 0.5 1.0

Voltage (V)

a b c

Potential (V vs. SCE)

CurrentDensity(Ag

1)

CurrentDensity(Ag

1)

0.5

0.4

0.3

0.2

0.1

0.0

0.1

0.2

0.3

0.4

0.5

1.5 2.0 1.0

3

2

1

0

1

2

3

0.80.60.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0

Fig. 59.15 Schematic representation of polyaniline formation on graphene surface via oxidative

polymerization of aniline by MnO2 (Reproduced with permission from Wu et al. (2010))


23/25


Fig. 59.16 Schematic illustration of two key steps for preparing hybrid graphene/MnO2-nanostructured textiles as high-performance EC electrodes. (i) Conformal coating of solution-

exfoliated graphene nanosheets (gray color) onto textile fibers. (ii) Controlled electrodeposition

of MnO2 nanoparticles (yellow dots) on graphene-wrapped textile fibers (Reproduced with

permission from Yu et al.2011)

(Wu et al.2010). These ECs can be cycled reversibly in the high voltage region of

02.0V (Fig. 59.15a). The resulting energy density of 30.4 Wh/kg is much higherthan those of the symmetric ECs based on graphene//graphene (2.8 Wh/kg)

(Fig. 59.15b) and MnO2graphene//MnO2graphene (5.2 Wh/kg) (Fig. 59.15c) and

higher than those of other MnO2-based asymmetric ECs. These findings open

up the possibility of the graphene-based composites for applications in safe

aqueous electrolyte-based high-voltage hybrids systems with high energy and power

densities.

Yu et al. (2011) demonstrated the solution-processed graphene/MnO2 nanos-

tructured textiles for the high-performance electrochemical capacitors applications.

In their study, solution-exfoliated graphene nanosheets (5 nm thickness) were

conformably coated on the three-dimensional, porous textiles support structures,and pseudocapacitive MnO2 nanomaterials was deposited by the controlled elec-

trodeposition (Fig. 59.16). This technique offers high loading of active electrode

materials and facilitates the easy access of electrolytes to those materials. The hybrid

graphene/MnO2-based textile yields high-capacitance performance with specific

capacitance up to 315 F/g. Also, they have fabricated asymmetric electrochemical

capacitors with the graphene/MnO2-textile as the positive electrode and single-

walled carbon nanotubes (SWNTs)-textile as the negative electrode in an aqueous

Na2SO4 electrolyte solution. These devices exhibit promising characteristics with

a maximum power density of 110 kW/kg, an energy density of 12.5 Wh/kg, andexcellent cycling performance of95 % capacitance retention over 5,000 cycles.

These kinds of low-cost, high-performance energy textiles-based nanostructures

offer great promise to realize the future large-scale energy storage devices.


24/25


5 Conclusions

In conclusion, this chapter began with the brief introduction of the advanced energy

devices such as lithium-ion batteries and high energy capacitors. The importance ofthe positive electrode materials and their synthesis by the solution process including

the hydrothermal and solvothermal method were discussed. These methods are

popularly used for the preparation of various inorganic materials owing to their

advantages such as size-controlled synthesis, morphology-controlled synthesis,

safety, easy synthesis, environmentally benign, and cost-effectiveness.

Among the lithium-ion batteries, cathodes are essential parts of the batteries.

The olivine-structured lithium metal phosphates are very much attractive due to

their cheap, environmentally friendly, and high theoretical capacity. The carbon

coating of lithium metal phosphates using different carbon sources via in situ or

ex-situ coating techniques has been discussed. The electrochemical property de-

pended on the morphology of LiFePO4and LiMnPO4cathodes has been discussed.

Most of the LiFePO4 nanomaterials less than 100250 nm in diameter exhibited

the discharge capacity close to the theoretical capacity (170 mAhg1). Thin plate

and rod morphology provides short diffusion length for the LiC-ion insertion and

exertion process. The discharge capacity of LiMnPO4 is not very impressive due

to its low intrinsic conductivity. New strategy development could improve the

electrochemical property of LiMnPO4for commercial purpose.

In the electrochemical capacitors, graphene-based compounds such as metal

oxide-doped graphene and PANI/graphene showed excellent capacitance whencompared to the other capacitors. In addition, asymmetric hybrid capacitors are

promising with higher capacitance for various powder density electric and electronic

devices. Further, continuous study of graphene could enable to understand its

physicochemical property for the electrochemical applications. New breakthrough

in these fields can change the performance of the energy devices and thus can make

human life more comfortable.

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